The goal of this research is to use functional Near Infrared Spectroscopy (fNIRS) to study the neural basis of irritability in a sample of preschool children (ages 3-5) who are being clinically treated for early onset psychopathology. Irritability exists on a dimension in every member of the population, however, it is also a significant mental health concern that is present in nearly every form of psychopathology. A new diagnostic category in the DSM-V revision has even been added in order to secure the research and clinical services that this symptom requires. The proposed study will be the first to examine brain mechanisms for frustration and inhibitory control, which signify key behavioral deficits, in young, highly irritable children. Obtaining these measures of functional abnormalities in the neural systems supporting frustration and inhibitory control in preschool children presenting with varying levels of irritability will first help us to identify which children show greater levels of dysfunction within these neural systems. It is also a first step toward the longer term goal of examining the extent to which these neural system abnormalities, identified at the age at which irritability first presents, may represent biological markers that can help distinguish relevant developmental trajectories of psychopathology, so that different treatments can be applied at this early stage of symptomatology. Children will be tested in the clinic setting using fNIRS, an optical imaging technique that is painless, non-restrictive, transportable, and allows for subject/experimenter interaction. Well-validated experimental paradigms will be employed. Primary analyses will probe the neural activity within specific regions related to frustration and inhibitory control, using state-of-the-art analysis techniques, in order to define brain deficits tat correlate with this symptom. Exploratory analyses will examine the extent to which these neural system abnormalities may predict symptom severity and clinical diagnosis by correlating activity in these neural systems with diagnostic report. Finally, long term plans for this research include a future proposal to complete a longitudinal R01 study, combining fNIRS and functional magnetic resonance imaging (fMRI). A large community sample of preschool children, across the range of irritability levels, will participate in yearly fNIRS measurement of neural systems fo frustration and inhibitory control using age appropriate tasks. In the later years of the project, when children have entered middle childhood, they will be scanned, using simultaneous fNIRS and fMRI to increase our understanding of the neural and clinical trajectories of this symptom. Our transdisciplinary research team unites developmental psychologists, neuroscientists, and child psychiatrists who are experts in their respective fields, but share a common interest in understanding the neural mechanisms for irritability as a predictor of child mental health outcomes.